Ah - direct monitoring is a trick I have never learnt. Can you guide?
The two basic methods of direct monitoring:
The second method (through the interface) can also be used without external preamps. Interfaces with DSP allow you to adjust the monitor signal tonally but will introduce a little latency. Most people don't care, but you are a phantastic player and I can imagine that you don't like to hear a different feeling than you had during the recording session.
Regarding the monitor mix – I don't like monitor controllers, for recording I prefer a mixer because it makes it easy to dial in a nice monitor sound for signal and playback. Mixers with reverb and compression are comfortable, maybe you have some hardware for effects. Alternatively you can take a reverb from the DAW: the latency becomes part of the predelay, which you need anyway.
... but how do I record piano plugin along with existing material?
Bad idea, you pass a couple of latency stages: A/D conversion + Computer + Logic/Plugins + D/A conversion + (without phones) distance from speaker to ear.
I have an old "Kurzweil MicroPiano" box and a small amp at the keyboard and select a sound similar to what I want. Such modules and amps are pretty cheap these days and give you practically zero latency. Additional benefit: your MIDI keyboard is playable without the computer.
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There is just one situation where direct monitoring can be a problem: Playing along a delay or other rhythmical effects. In this cases you may be forced to make your DAW as fast as possible and use it for monitoring.
Another (better) way would be to use a good hardware delay and record it on a second track. While you modify notes or change the arrangement, you can run the dry track via Aux and I/O plugin through the same hardware, finally record the hardware to it's own track.